is it the gingerbread BAMF? cause if u came from froyo, and installed a gb rom, u wont have service (cuz u have the froyo radio), and need to flash a gb radio
Download Rom Manager and open, flash clockwork recovery,
Put the GB Rom of your choice (named whatever.zip),
and the GB radio (named PG05IMG.zip) on the root (not in any folders) of your sdcard,
boot to recovery,
make a backup,
when that's done wipe data/factory reset x2,
install zip from sdcard, choose zip,
find the Rom from the list,
when it's done,
battery pull,
boot phone while holding down on volume,
a white screen will appear and scan for the PG05IMG.zip,
press volume up when it says to update,
reboot when done.